Model of 2 joined mirrored objects is missing faces when importing into Unity?
This sort of issues happens when the mirror modifier has the clipping option turned off and is not merging overlapping vertices.
the different color on those faces indicate that those faces are flipped.
it's a bit tricky to detect hidden geometry but your mesh looks like it's made of 12 faces when at the top you can see there are 14 (2 hidden inside the mesh).

delete the internal faces : turn on wireframe view mode to see them and dissolve them.
remove the doubles vertices : select all the geometry ,press  W and select remove doubles.
recalculate the normals : shortcut ctrlN